Volumetric metering devices are used widely in many different technical fields in which there is a need to deliver a substantially continuous and measured flow of material over time. For example, the technical field of agricultural sowing machines with pneumatic seed-transportation is typical; in these machines it is known to provide a seed-loading hopper at the base of which a metering device, generally of the volumetric type with blades, is fitted and supplies metered quantities of seeds into a Venturi duct by which the seeds are admitted to an air-flow and transported to the sowing drills.
The particular and preferred application of the invention is in all cases in which it is necessary to vary the flow-rate delivered, possibly within a fairly large range of values.
Typical metering devices formed in accordance with the prior art are described, for example, in EP 0702890, EP 0606541, EP 0471155, and U.S. Pat. No. 6,138,591.
Conventional metering devices normally comprise a box-like structure in which a bladed metering member is supported rotatably and cooperates with a flow-restricting device for regulating the flow-rate delivered. In one example, the restricting device comprises a jacket movable axially in an adjustable manner relative to the rotor of the distributor so as to cover a portion of the set of blades in order to reduce the overall capacity of the cells available. Alternatively, the rate of rotation of the metering member may be varied, or the capacity of the cells may be varied.
A first problem resulting from the known structure of conventional distributors is that they do not permit large adjustments of flow-rate whilst maintaining constant regulation of the metering for each adjustment. In fact, there are critical speeds of the metering member beyond which its reliability is reduced.
A second problem typical of known structures is that they comprise a plurality of parts which have to be assembled using connections, screws, flanges, etc. This construction is rather unsuitable for agricultural machines for which many adjustments have to be performed in the field and the consequent need to remove various parts may lead to their loss, making re-assembly impossible.
A further problem characteristic of conventional metering devices is that they are subject to irremediable damage, or at least to blockage of the metering member, when foreign bodies of considerable size reach the rotor. In the absence of a timely indication of this situation, particularly during sowing, it is possible, completely unwittingly, to sow whole fields unevenly or not at all.
An object of the present invention is to provide an improved volumetric metering device that solves one or more of the problems identified above.